Networking
• A few questions on the course
• A few questions about the course
The Head of Department is
Pet
er J
ohnso
n
Chris
Jen
nison
Gly
nis B
reak
wel
l
Phil
Will
is
Kev
in E
dge
44%
8%11%
31%
6%
A. Peter Johnson
B. Chris Jennison
C. Glynis Breakwell
D. Phil Willis
E. Kevin Edge
That was the “warmup”
• I hope you got Phil Willis:
• Peter Johnson is so last last year!
• Chris is Phil’s boss, Kevin is Chris’s boss, and Glynis is Kevin’s boss.
Questions on the course
(mostly factual, but there is more to the course than facts)
An Ethernet address is how long?
Var
iabl
e le
ngth
16
byte
s
6 b
ytes
4 b
ytes
3%
18%
43%38%
A. Variable length
B. 16 bytes
C. 6 bytes
D. 4 bytes
The answer is C (6 bytes)
If you didn’t get that, you’re pretty confused: After last time, all I can suggest is that you really re-read sections 2.2 and 3.2 of the book.
Which of these is not a well-known routing protocol?
Routin
g Info
rmat
ion ..
.
Open
Shorte
st P
ath F
irst
Route
r Dis
cove
ry P
r...
Bord
er G
atew
ay P
ro...
8% 5%
55%
33%
A. Routing Information Protocol
B. Open Shortest Path First
C. Router Discovery Protocol
D. Border Gateway Protocol
C — Router Discovery Protocol
A. RIP is a well-known protocol
B. OSPF is a well-known protocol
C. RDP, as far as I know, doesn’t exist: Router Discovery Messages (chapter 9) say who routers are.
D. BGP is a well-known protocol
There are thee kinds of autonomous system: which of
these is not one:
Stu
b
Confe
dera
tion
Tra
nsit
Multi
hom
ed
2%
16%16%
66%
A. Stub
B. Confederation
C. Transit
D. Multihomed
B (Confederation) is not a kind of autonomous system
If you got this wrong, I suggest section 10.7 of the book, and also
the updating notes for that
Which one of these statements about IP fragmentation is true
A fr
agm
ente
d pac
ket..
.
Fra
gmen
ts a
re p
ut to.
..
Fra
gmen
tatio
n is
opt..
.
Fra
gmen
tatio
n is
wh..
It is
vita
l that
frag
men
..
2%11%
7%7%
73%
A. A fragmented packet can never be refragmented
B. Fragments are put together as soon as possible
C. Fragmentation is optional for the sender
D. Fragmentation is what happened to officers in Vietnam
E. It is vital that fragments arrive in the right order
C (optional) is the right answer
A. fragmented packets can be re-fragmented
B. The receiver (or its firewall) puts fragments together, not intermediate hosts
C. True: set the DF flagD. That’s not IP fragmentationE. Fragments can arrive in any order: the
offset field tells how to put them back
Which one of these is true
255
.255
.255
.255
is c
all..
Bro
adca
stin
g is h
ow ..
.
Giv
en th
e ch
oice,
mul..
.
Multi
cast
ing o
nly w
or..
20%16%
56%
9%
A. 255.255.255.255 is called limited broad-cast because it’s only used on startup
B. Broadcasting is how internet radio works
C. Given the choice, multicast is generally better than broadcast
D. Multicasting only works on Ethernets
The answer was C: multicast is generally better than broadcast
A. It’s called limited because routers don’t forward it, so it’s limited to that net
B. Internet radio etc. work on multicasting
C. Yes – doesn’t load those who aren’t interested
D. No: multicasting is very important on wide-area networks (e.g. radio)
Between A and PTR records
The
DNS is a
lway
s c.
..
Consi
sten
t exc
ept f
or...
Mig
ht wel
l be
inco
nsi...
Consi
sten
cy is
mea
n...
Consi
sten
cy is
unde.
..
14%
27%
20%18%
20%
A. The DNS is always consistent
B. Consistent except for deliberate fraud
C. Might well be inconsistent
D. Consistency is meaningless
E. Consistency is undecidable
A “thinking” question: C or E
A. Certainly not true
B. Fraud can certainly work by adding inconsistencies, but it’s not the only way
C. DNS maintainers can easily screw up!
D. An abstract definition is possible
E. But it requires testing infinitely many possible cases. Also the DNS is distributed, so untestable.
The DNS …
Alw
ays
uses
UDP
Alw
ays
uses
TCP
Norm
ally
use
s UDP
Ques
tions
are
in U
D..
20% 23%
43%
14%
A. Always uses UDP
B. Always uses TCP
C. Normally uses UDP
D. Questions are in UDP, but replies can be in either depending on length.
C — normally UDP
• Both can be used
• UDP is more common for most transactions
• Hence C is the answer
• A UDP question always gets a UDP answer, possibly marked TC (truncated). It can be re-asked in TCP if necessary
Which one of these statements about TFTP is true
TFTP w
as in
vente
d b...
TFTP, l
ike
UDP, is
unr...
TFTP is
obso
lete
TFTP u
ses
UDP for s
...
16%
56%
9%
19%
A. TFTP was invented before TCP, so uses UDP
B. TFTP, like UDP, is unreliable
C. TFTP is obsolete
D. TFTP uses UDP for simplicity
D — simplicity
A. No – TCP is RFC 793 but TFTP is RFC 1350. Anyway date isn’t the reason
B. TFTP numbers packets, and asks for repeats, so is reliable
C. TFTP is in daily use in the Library
D. Simplicity is the reason: UDP is simple, unchanging and fits in a boot PROM.
The TCP header is
Exa
ctly
20
bytes
long
At l
east
20
bytes
long
At m
ost 2
0 by
tes
long
Exa
ctly
60
bytes
long
Bet
wee
n 20
and 6
0 b...
34%
14%
36%
2%
14%
A. Exactly 20 bytes long
B. At least 20 bytes long
C. At most 20 bytes long
D. Exactly 60 bytes long
E. Between 20 and 60 bytes long
E (between 20 and 60)
A. 20 is the minimum
B. Correct, but not the whole truth
C. Wrong
D. 60 is the maximum, but it’s nearly always less
E. Correct
The ACK bit in the TCP header is set
All
the
time
Only
when
ther
e’s
ne...
On e
very
pac
ket e
xce.
.
On e
very
pac
ket e
xcep
...
0%
16%
59%
25%
A. All the timeB. Only when there’s
new data being acknowledged
C. On every packet except the first
D. On every packet except the first in each direction
C — every packet except first
A. Can’t be set on first packet — there’s nothing to acknowledge
B. The data might not seem new to us, but our previous acknowledgement might have been lost
C. Correct
D. No — the first packet from B to A has to acknowledge A’s SYN
A TCP segment with SYN must have a byte of data, so that it can
be acknowledged
Tru
e
Fal
se
42%
58%
1. True
2. False
False
The SYN bit itself is counted as if it were a byte of data. It is this “pretend” byte that is acknowledged.
The initiator of a TCP connection must be the one to close it.
Tru
e
Fal
se
93%
7%
1. True
2. False
False
In a typical client-server situation, it is often the client (which is the one who starts the connection) who finishes it, but this is not an invariable rule. HTTP is a counter-example (at least in HTTP 0.9/1.0)
Networking
• A few questions on the course
• A few questions about the course (starting with some comments from the survey some of you kindly did)
Good fun, something different and its very good to be interactive since we do not get much class participation, except
if we want to ask questions
Stro
ngly A
gree
Agre
e
Neu
tral
Dis
agre
e
Stro
ngly D
isag
ree
54%
39%
2%0%5%
1. Strongly Agree
2. Agree
3. Neutral
4. Disagree
5. Strongly Disagree
Although good for general feedback how we are doing, it’s not so good
for passing course material on.
Stro
ngly A
gree
Agre
e
Neu
tral
Dis
agre
e
Stro
ngly D
isag
ree
11%
34%
5%
27%
23%
1. Strongly Agree
2. Agree
3. Neutral
4. Disagree
5. Strongly Disagree
I wish we’d had more networking earlier in the degree course
Stro
ngly A
gree
Agre
e
Neu
tral
Dis
agre
e
Stro
ngly D
isag
ree
23%
40%
19%
12%
7%
1. Strongly Agree
2. Agree
3. Neutral
4. Disagree
5. Strongly Disagree
I wish this Audience Response System had been used earlier
(i.e. in previous years)
Stro
ngly A
gree
Agre
e
Neu
tral
Dis
agre
e
Stro
ngly D
isag
ree
62%
26%
2%0%
10%
1. Strongly Agree
2. Agree
3. Neutral
4. Disagree
5. Strongly Disagree